Optical apparatus using a phase grating

ABSTRACT

An optical apparatus comprises a phase grating, an input-ring array, an output-ring array and an optical interconnect. The phase grating produces a modulated light beam for each of a plurality of processing elements. The modulated light beams have a plurality of light wavelengths. An input-ring array receives the plurality of data modulated light beams. An output-ring array outputs a light beam to each processing element. An optical interconnect transfers light from the input-ring array to the output ring array and uniquely rotates each wavelength of the transferred light.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 10/162,262, filed Jun. 4, 2002 which is a continuation of U.S.patent application Ser. No. 10/067,015, filed Feb. 4, 2002, which issuedon Jul. 30, 2002 as U.S. Pat. No. 6,426,828, which is a continuation ofU.S. patent application Ser. No. 09/847,180, filed May 2, 2001, whichissued on Apr. 16, 2002 as U.S. Pat. No. 6,373,605, which is acontinuation of U.S. patent application Ser. No. 09/430,543, filed Oct.29, 1999, which issued on Jun. 5, 2001 as U.S. Pat. No. 6,243,180, whichis a continuation of U.S. patent application Ser. No. 08/896,367, filedJul. 18, 1997, which issued on Dec. 28, 1999 as U.S. Pat. No. 6,008,918,which is a continuation of U.S. patent application Ser. No. 08/641,632,filed May 2, 1996, which issued on Oct. 14, 1997 as U.S. Pat. No.5,677,778, which is a continuation of U.S. patent application Ser. No.08/068,518, filed May 28, 1993, which issued on May 7, 1996 as U.S. Pat.No. 5,515,194, all of which are incorporated herein by reference.

BACKGROUND

[0002] This invention relates to an optical free space interconnect forultra-high speed single instruction multiple data processors.

[0003] As the geometries of VLSI grow smaller and denser electronicinterconnects and heat dissipation have been recognized as bottlenecksof advanced electronic computing systems. F. E. Kiamilev and et al.,PERFORMANCE COMPARISON BETWEEN OPTOELECTRONIC AND VLSI MULTISTAGEINTERCONNECTION NETWORKS, J. Lightwave Tech., vol. 9, pp. 1674-1692,1991. M. R. Feldman, S. C. Esener, C. C. Guest, and S. H. Lee,COMPARISON BETWEEN OPTICAL AND ELECTRIC INTERCONNECTS BASED ON POWER ANDSPEED CONSIDERATIONS, Appl, Opt., vol. 27, pp. 1742-1751, 1988.

[0004] Furthermore, as systems are operated at higher and higher speeds,the latency induced by electronic connections becomes a limiting factor.Although some new techniques, such as three dimensional multi-chipmodules, have been developed to provide short connection distances andless latency, the basic limitation of the pin-out problem in electronicconnections cannot be fully removed. L. D. Hutchson and P. Haugen,OPTICAL INTERCONNECTS REPLACE HARDWIRE, IEEE Spectrum., pp. 30-35, 1987.

[0005] Optical interconnections, because of their three-dimensional(3-D) processing capabilities and matched impedance characteristic, havebeen considered as the best alternative to electronic interconnections.Optical implementations of chip-to-chip and backplane-to-backplaneinterconnections have been reported. See, e.g. J. W. Goodman, OPTICALINTERCONNECTIONS FOR VLSI SYSTEMS, Proc. IEEE, vol. 72, p. 850, 1984.Optical 3-D multi-stage interconnection networks have been investigatedand realized. A. A. Sawchuk Proc. SPIE, vol. 813, p. 547, 1987.Recently, a new free space optical interconnect based on ring topologieswas proposed by Y. Li, B. Ha, T. W. Wang, A. Katz, X. J. Lu, and E.Kanterakis Appl, Opt., vol. 31, p. 5548, 1992.

[0006] Arranging the input array on a ring, this novel architecture iscapable of interconnecting many processors with identical latency andminimal complexity, and cost. This architecture is best suitable for theimplementation of Single Instruction Multiple Data (SIMD) streammachines. Most topologies developed for rectangular array, such asNearest Neighbor (NN), Plus Minus 2I (PM2I), and Hypercube can beimplemented using this ring is topology. A simplified architecture ofthe ring topology architecture is depicted in FIG. 1. The opticalinterconnect has an input ring array 42 and an output ring array 45. Theoptical interconnect uses a first plurality of beamsplitters 23, 24, 25,26, connected to the input ring array 42. The first plurality ofbeamsplitters 23, 24, 25, 26 is connected through a plurality of opticalswitches 27, 28, 29, 30, a plurality of dove prisms 56, 57, 58, 59 to asecond plurality of beamsplitters 61, 62, 63, 64. A multiple channelsystem is used to perform a certain interconnection, i.e. a set ofpermutations. The number of channels depends on the topology to berealized. For example, to realize the NN type of interconnect shown inFIG. 2, a four channel system is needed.

SUMMARY

[0007] An optical apparatus comprises a phase grating, an input-ringarray, an output-ring array and an optical interconnect. The phasegrating produces a modulated light beam for each of a plurality ofprocessing elements. The modulated light beams have a plurality of lightwavelengths. An input-ring array receives the plurality of datamodulated light beams. An output-ring array outputs a light beam to eachprocessing element. An optical interconnect transfers light from theinput-ring array to the output ring array and uniquely rotates eachwavelength of the transferred light.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0008] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate preferred embodimentsof the invention, and together with the description serve to explain theprinciples of the invention.

[0009]FIG. 1 shows general structure of the originally proposed opticalring topology NN interconnect architecture;

[0010]FIG. 2 shows grid array of an NN interconnect architecture;

[0011]FIG. 3 shows a wavelength selection, external modulation system;

[0012]FIG. 4 shows the structure for an optical ring topology NNinterconnect architecture;

[0013]FIG. 5 shows electronically controlled tunable laser;

[0014]FIG. 6 shows general architecture of an interferometricMach-Zehnder switch;

[0015]FIG. 7 shows a free space power combiner;

[0016]FIG. 8a shows an alternate free space power splitter;

[0017]FIG. 8b shows a cross section at focal plane;

[0018]FIG. 9 illustrates data modulation and coupling to input ring; and

[0019]FIG. 10 shows spectral selectivity of a reflection holographicgrating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0020] Reference now is made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

[0021] The present invention provides a novel ring topology basedarchitecture. The major features of this architecture are:

[0022] 1. Instead of direct modulation of the laser diode current,external modulation may be used. Under this approach, integrated singleelement electro-optic switches capable of operating in excess of 40 GHzmay be used.

[0023] 2. Multiple wavelength laser diode sources combined with externalswitches are used to control the routing of light to different channels.This eliminates the need of two dimensional (2-D) array switchingdevices. The system performance may no longer be limited by thesedevices.

[0024] 3. Wavelength selective holographic optical elements(holographic-optical elements) may be used to form the routing functionof the system based on the selected light wavelength.

[0025] Broadly, the present invention uses a modulation system connectedto an optical interconnect. The modulation system is external to theoptical interconnect. The modulation system provides wavelengthselection, and modulates the selected wavelength with data. Broadly, themodulation system includes second generating means, selecting means,combining means, dividing means, modulating means, and forming means.The selecting means is coupled to the first generating means and to thesecond generating means. The combining means is coupled to the selectingmeans. The dividing means is coupled to the combining is coupled to theselecting means. The dividing means is coupled to the combining means.The modulating means is coupled to the plurality of SIMD processors, andto the dividing means. The forming means is coupled to the modulatingmeans.

[0026] The second generating means generates a plurality ofcoherent-light beams at a plurality of wavelengths. In response to acontrol signal from the first generating means, the first selectingmeans selects a single coherent-light beam at a single wavelength fromthe second generating means. The combining means combines output lightfrom the selecting means. The dividing means divides the output lightinto a plurality of equal-length paths. Each equal-length pathcorresponds to each of the plurality of SIMD processors. Using data fromthe appropriate SIMD processors, the modulating means modulates theoutput light. The forming means forms an input ring at an input plane ofthe optical interconnect.

[0027] In the exemplary arrangement shown in FIG. 3, an opticalinterconnect for a plurality of single-instruction-multiple-data (SIMD)processors is provided using the first generating means. The firstgenerating means is embodied as a controller 21 or equivalently acentral processing unit (CPU). The CPU may employ a computer using, byway of example, 8086, 80286, 80386, 80486, or Pentium microprocessor, orequivalently a 68040 microprocessor, a larger computer system, or anapplication specific integrated circuit (ASIC) designed for the opticalinterconnect.

[0028] The second generating means is embodied as a plurality of laserdiodes 31, 32, 33, 34, for generating a plurality of coherent lightbeams at a plurality of wavelengths, respectively. Each of the pluralityof laser diodes 31, 32, 33, 34, operates at a different wavelength.

[0029] The selecting means is embodied as a first plurality ofoptical-switching devices 36, 37, 38, 39. The first plurality ofoptical-switching devices 36, 37, 38, 39 are coupled to a plurality oflaser diodes 31, 32, 33, 34, respectively. The optical-switching devicescontrol the on/off output of coherent-light beam from their respectivelaser diodes. Thus, a first optical switching device 36 controls whetheror not the light from laser diode 31 is radiated. Similarly, secondoptical switching device 37 controls whether or not the light from laserdiode 32 is radiated. Generally, the plurality of laser diodes 31, 32,33, 34 radiate light continuously. The first plurality ofoptical-switching devices 36, 37, 38, 39 is coupled to the controller 21through control bus 22. The controller 21 controls the operation of eachof the first plurality of optical-switching devices 36, 37, 38, 39.

[0030] The combining means is embodied as a fiber combiner and splitter40. The fiber combiner and splitter 40 is connected to the plurality ofoptical-switching devices 36, 37, 38, 39. The fiber combiner andsplitter 40 combines output light from the first plurality ofoptical-switching devices 36, 37, 38, 39. An optical fiber, included inthe fiber combiner and splitter 40, is connected between the fibercombiner and the fiber splitter for carrying the output light from thefiber combiner to the fiber splitter. The fiber splitter, which isconnected at the output of the optical fiber, divides the output lightinto a plurality of equal-length paths. Each equal-length pathcorresponds to one of the plurality of SIMD processors 25. Thus, theoutput light from the optical-switching devices is fed to a single fiberwith the use of a fiber combiner. The optical-switching devices arecontrolled by a controller 21 so that at each clock cycle, only onewavelength is selected. Therefore, only one light wavelength is presentat the output of the fiber combiner. The fiber output is divided by afiber splitter into a number of N equal length paths corresponding tothe number of processors in the system. Each path serves one processor.

[0031] The modulating means is embodied as a second plurality ofoptical-switching devices 49. The second plurality of optical-switchingdevices 49 is coupled to the output of the fiber combiner and splitter40, and to the plurality of SIMD processors 25 through data bus 26. Thesecond plurality of optical-switching devices 49 modulates the outputlight from the fiber combiner and splitter 40.

[0032] Forming means may be embodied as a plurality of optical fibers.The plurality of optical fibers is connected to the second plurality ofoptical-switching devices 49. The plurality of optical fibers forms aninput ring 42 at an input plane of the optical interconnect. Also shownon FIG. 3, is the output of the optical interconnect which is anoutput-ring array of optical detectors connected to the SIMD processors25 through data bus 27. The plurality of SIMD processors 25 arecontrolled by controller 21 through control bus 23.

[0033] Thus, data modulation is performed by placing the secondplurality of optical-switching devices 49 onto each output of the fibersplitter. The output fibers from the second plurality ofoptical-switching devices 49 are used to form the input ring at theinput plane of the free space optical interconnect. The function of theoptical system is to route the data through the proper channel accordingto the selected light wavelength. The output data from the interconnectsystem is then fed back to the Processing Elements (PEs), referred toherein as the SIMD processors, of the digital system.

[0034] As illustratively shown in FIG. 4, the structure for the opticalring topology interconnect architecture is depicted. The input system isconnected to the input ring 42, and output detectors, such asphotodiodes, form an output ring array 43. The optical interconnectincludes first reflecting means, image-rotating means, and secondreflecting means. The first reflecting means is depicted as a pluralityof holographic-optical element reflectors 51, 52, 53, 54. Theimage-rotating means is depicted as a plurality of dove prisms 56, 57,58, 59. The second reflecting means is shown as a plurality ofbeamsplitters 61, 62, 63, 64. The initial beam splitter 61 need only bea mirror. The plurality of dove prisms 56, 57, 58, 59 are connectedbetween the plurality of holographic-optical elements 51, 52, 53, 54 andthe plurality of beamsplitters 61, 62, 63, 64, respectively.Accordingly, a first dove prism 56 is connected between a firstholographic element 51 and either a first beam splitter 61 orequivalently a mirror. The second dove prism 57 is connected between asecond holographic-optical element 52 and the second beam splitter 62.The third dove prism 58 is connected between the thirdholographic-optical element 53 and the third beam splitter 63. Eachholographic-optical element reflects light at one wavelength and istransparent to light at other wavelengths. Light passing through each ofthe dove prisms is at a wavelength according to the holographic-opticalelement corresponding to that dove prism. The beamsplitters 61, 62, 63,64 reflect light at all wavelengths passing through the dove prisms andtheir respective holographic-optical elements.

[0035] The plurality of dove prisms 56, 57, 58, 59 are fixed at aspecific orientation, i.e. rotation, with respect to the input ring.Accordingly, a dove prism performs a selected interconnection betweenSIMD processors according to the orientation of the dove prism, rotationabout the optical axis of the dove prism, i.e., with respect to theinput ring. The output optical ring is connected to the plurality ofSIMD processors 25.

[0036] In use, the plurality of laser diodes 31, 32, 33, 34 generate aplurality of coherent-light beams at a plurality of wavelengths. Each ofthe coherent-light beams is at a different wavelength from the othercoherent-light beams of the plurality of coherent-light beams. The firstplurality of optical-switching devices 36, 37, 38, 39 are controlled bya control signal from the controller 21. The control signal determineswhich of the optical-switching devices allows light to pass from theplurality of laser diodes 31, 32, 33, 34. Accordingly, by theappropriate control signal, the first plurality of optical-switchingdevices selects a single coherent-light beam from the plurality ofcoherent-light beams, at a single wavelength. The fiber combiner andsplitter 40 combines output light from the first plurality ofoptical-switching devices 36, 37, 38, 39, and divides the output lightinto a plurality of equal-length paths. Each of the equal-length pathcorresponds to each of the plurality of SIMD processors.

[0037] Data from the SIMD processors, at a given moment in time,modulates the output lights from the fiber combiner and splitter 40. Thecontroller 21 controls which of the SIMD processors 25 modulates theoutput light. The plurality of optical fibers forms an input ring at theinput plane of the optical interconnect, so that output light at anywavelength has the same path in length into the optical interconnect.

[0038] In the optical interconnect, each of the plurality ofholographic-optical elements 51, 52, 53, 54 reflects light at a singlewavelength and is transparent to light at other wavelengths. Therefore,only one optical channel is operational for each wavelength. Selectedpermutations are accomplished by controlling the optical-switchingdevices 36, 37, 38, 39. Clearly only one of them is on at a time.Absorption and diffraction efficiency of the holographic-opticalelements are the two factors which may induce power loss in the freespace optical interconnect system. Absorption in the infrared region isvery low for photopolymer holographic materials. A diffractionefficiency of more than 99% has been obtained for a reflection typehologram. Therefore, the free space optical system may attain nearly100% light efficiency. The power loss due to having all lasers operatingat all times may be alleviated by using a single tunable laser diode 66,see FIG. 5. However, this puts restrictions on the switching speedbetween wavelengths and the selectivity of the holographic-opticalelements.

[0039] The plurality of dove prisms 56, 57, 58, 59 each perform at agiven time, a selected fixed interconnection between SIMD processorsaccording to the orientation of the dove prism, rotation about theoptical axis of the dove prism, i.e., with respect to the input ring.The optical channel is determined by the control signal from thecontroller 21 which controls the first plurality optical-switchingdevices 36, 37, 38, 39. The plurality of beamsplitters 61, 62, 63, 64are coupled to the plurality of dove prisms 56, 57, 58, 59 and reflectthe light from the selected optical channel to the output-ring detectorarray 43. The output ring detector array 43 detects the output lightfrom the selected optical channel and the data are fed on the data bus27 to the SIMD processors 25.

[0040] The channel selecting means alternatively may be embodied asinternal modulators to the lasers. The light admitted by the lasers thusis modulated by altering the electric current which drives each laser.While FIG. 3 shows external light modulation, internal modulation mayalso be used depending on the processor speed and may be sufficient.

[0041]FIG. 7 illustratively shows the light admitted by all lasers maybe combined and then split approximately at equal powers to a set ofcomponents the number of which equals the number of processing elements.There are many alternatives in performing this function.

[0042]FIG. 3 illustrates an efficient method when a small number ofprocessors are involved. For a large number of processors, other moreefficient methods as the one illustrated in FIGS. 7 and 8 may beemployed.

[0043]FIG. 8a shows an alternative free space power splitting systemwhich includes phase grating 86 and optical system 87. A cross sectionat focal plane 88 is shown in FIG. 8b as a grid array of line spectra.

[0044]FIG. 9 shows the data modulation and coupling to the input ring 44which may employ a 2-D spatial light modulator 91 coupled to the datafrom the processing elements 25. Free space to fiber optical system 92is coupled to the 2-D spatial light modulator and the input ring 44.

[0045] A complex component in the system is the one containing the datamodulators, i.e. the optical-switching devices. Although differentswitching devices may be used, waveguide switching devices are wellsuitable for this purpose. Waveguide switches have been investigated andused in optical integrated circuits for almost twenty years. See L.Thylen IEEE J. Lightwave Technol., vol. 6, p. 847, 1988. The most commontype of waveguide switches is the interferometric Mach-Zehnder (M-Z)switch shown in FIG. 6. Light is coupled into a single-node fiber, andthen split evenly into two paths forming a Y junction.

[0046] The light from the two paths is then combined into a single modewaveguide 67. Depending on the relative phase delay between the twooptical paths, the recombined light output can be switched from amaximum to a minimum power. This phase delay can be controlled by anelectronically applied voltage. M-Z devices can operate at anywavelength at which the waveguide is transparent, and thus areappropriate for use in the system architecture. M-Z devices can be alsobe used as a data modulator. The maximum data rates at which the systemcan operate is not limited by the bandwidth of M-Z devices. Currently,LiNbO₃ based semiconductor waveguide M-Z switches have been operated at40 GHz while maintaining greater than 20-dB on/off extinction ratio.Electro-optic polymers may also be candidates for fast speed switchingdevices because of their compatibility with optoelectronic integration.

[0047] The holographic-optical elements are key components in thisinterconnect system. The requirements of the holographic-opticalelements are high diffraction efficiency, high spectral selectivity, andlow absorption. The holographic-optical elements shown in FIG. 4 arereflection type holograms, though both reflection and transmission typeholograms may be used.

[0048] Because of their higher wavelength selectivity, reflectionholograms are desirable. To estimate the wavelength selectivity of areflection hologram, a brief analysis is given. According to Kogelnik H.Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst.Tech. J., vol 48, pp. 2909-2947, 1969, the amplitude of the diffractedwave for an unslanted reflection grating is expressed by $\begin{matrix}{{{S = \frac{- i}{( {{\frac{i}{\xi_{r}}\gamma_{r}} + {\lbrack {1 - ( {\xi_{r}\gamma_{r}} )^{1/2}} \rbrack^{1/2}{\coth ( {\gamma_{r}^{2} - \xi_{r}^{2}} )}^{1/2}}} }},{where}}} & (1) \\{{\upsilon_{r} = {\pi \quad n_{1}\frac{T}{\lambda sin\theta}}},} & (2) \\{{\xi = {{{\delta\beta}\quad T\quad \cos \quad \theta} = {\frac{2\pi}{\lambda}n_{0}\delta \quad T\quad \cos \quad \theta}}},} & (3)\end{matrix}$

[0049] δ is the deviation of the incident angle away from the Braggangle θ₀ given by δ=θ−θ₀. T is the thickness of the recording material,λ is the illumination wavelength, n₁ is the index modulation, n₀ is theindex of refraction of the material, and θ is the incident angle. TheBragg condition is expressed by

2dn ₀ sin θ₀=λ₀,  (4)

[0050] where d is the grating spacing and λ₀ is the wavelengthsatisfying the Bragg condition. Under Bragg condition, maximumdiffraction efficiency is obtained. Letting the wavelength of theillumination be changed to λ₀+δλ₀, the maximum diffraction efficiency isno longer obtained at the illumination angle θ₀, but instead is given bythe new Bragg angle θ′₀=θ₀+δθ. If we maintain the illumination at theoriginal angle θ₀=θ′₀−δθ, diffraction efficiency is reduced accordingthe deviation δθ from the Bragg angle θ′₀. The spectral selectivity isdefined as the width δλ until the diffraction efficiency goes to zero.The Bragg condition given in Equation 4 may be expressed in terms of δλas

2dn ₀ sin (θ₀+δθ)=λ₀+δλ,  (5)

[0051] Making some approximations and using Equation 4, yields$\begin{matrix}{{\delta\theta}_{0} = {{- \frac{\delta\lambda}{\lambda}}\tan \quad \theta_{0}}} & (6)\end{matrix}$

[0052] Substituting the above equations into Equation 1, the formula forthe diffraction efficiency is obtained. FIG. 10 illustrates thediffraction efficiency dB vs. δλ. The parameters used were λ=1.3 μm,T=500 μm, n=1.52, θ=π/4, and υ=0.95 π. The curve shows that for theseparameters, the spectral selectivity is equal to 1.2 nm. Whenholographic-optical elements are used, crosstalk is induced by lightfrom the zeroth order. In the above case, the crosstalk is less than −20dB, which is equivalent to a contrast ratio of 100:1 in an intensityswitching configuration. Evidently, the high wavelength selectivity andthe low crosstalk will allow us to employ a large number of channels.This architecture, while maintaining all the advantages of past ringtopologies, overcomes current switching device problems such as slowspeed and lack of appropriate 2-D array devices. Employing waveguideswitches, the system is capable to operate at data rates up to 40 GHz.The use of holographic-optical elements makes possible to self-route thewavelength coded input data. With the development of waveguide switches,the proposed architecture leads to a very powerful interconnect systemfor the implementation of SIMD machines.

[0053] To ensure the full functionality of the ring topology basedoptical free space interconnect system, device issues should beaddressed. Two basic devices are needed in the implementation of thisarchitecture: optical transmitters/receivers and optical switchingdevices. Theoretically, since the system has identical latency, theinterconnect system does not impose any limitation on the system datarate(s).

[0054] The present invention also includes a method for opticallyinterconnecting a plurality of SIMD processors. The method includes thesteps of generating a control signal from a processor, and generating aplurality of coherent-light beams at a plurality of wavelengths,respectively. In response to the control signal, the method selects afirst coherent-light beam at a first wavelength from a plurality ofwavelengths, and combines and divides the output light for each of theplurality of wavelengths into a plurality of equal-length paths. Each ofthe equal-length paths corresponds to each of the SIMD processors. Usingdata from a respective SIMD processor, the method modulates,corresponding to the light with selected wavelength, the output light,and forms with the output light at a respective wavelength, and inputring at the optical interconnect.

[0055] The method includes using the optical interconnect for reflectingfrom the input plane the output light, and in response to the controlsignal, selecting an optical channel for the reflected output light. Thelight from the selected optical channel is reflected to an output-ringdetector array, and is thereby detected by the detectors. The detectedsignal is fed back to the SIMD processors.

[0056] The method performs the above steps for each wavelength, and thenrepeats the entire sequence for all wavelengths.

[0057] It will be apparent to those skilled in the art that variousmodifications can be made to the optical interconnect for high speedprocessors of the instant invention without departing from the scope orspirit of the invention, and it is intended that the present inventioncover modifications and variations of the optical interconnect for highspeed processors provided they come within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. An optical apparatus comprising: a phase gratingfor producing a modulated light beam for each of a plurality ofprocessing elements, the modulated light beams having a plurality oflight wavelengths; an input-ring array for receiving the plurality ofdata modulated light beams; an output-ring array for outputting a lightbeam to each processing element; and an optical interconnect fortransferring light from the input-ring array to the output-ring arrayand uniquely rotating each wavelength of the transferred light.
 2. Theoptical apparatus of claim 1 comprising a free space to fiber opticsystem coupled to a plurality of optical fibers for receiving themodulated light beams produced by the phase grating and outputting themodulated light beams to the input-ring array.
 3. The optical apparatusof claim 1 wherein the uniquely rotating each wavelength is performed bya plurality of prisms and beam splitters.
 4. The optical apparatus ofclaim 1 wherein the optical interconnect comprises a first opticalelement reflector for reflecting one of the plurality of lightwavelengths, a prism for rotating the one wavelength, and a secondoptical element reflector for directing the rotated one wavelength tothe output-ring array.
 5. A method for transferring data between aplurality of processors, the method comprising: providing a phasegrating; producing using the phase grating a modulated light beam foreach of a plurality of processing elements, the modulated light beamshaving a plurality of light wavelengths; receiving at an input-ringarray the plurality of data modulated light beams; outputting to anoutput-ring array a light beam to each processing element; andtransferring light from the input-ring array to the output-ring arrayand uniquely rotating each wavelength of the transferred light.
 6. Themethod of claim 5 wherein the transferring light is performed by anoptical interconnect having a plurality of prisms and beamsplitters. 7.The method of claim 5 further comprising providing a first opticalelement reflector for reflecting one of the plurality of lightwavelengths, a prism for rotating the one wavelength, and a secondoptical element reflector for directing the rotated one wavelength tothe output-ring array.